The invention relates to the field of precision timing distribution which is especially critical for future accelerator facilities, and in particular for precise synchronization between low-level RF-systems in such facilities as well as to the extraction of microwave signals from optical clocks.
Seeding of free electron lasers operating in the EUV and soft X-ray regime with radiation generated via high harmonics from noble gases may result in a fully coherent X-ray laser. For seeding of such large-scale facilities spanning over several hundreds meters, it is critical to synchronize lasers and RF-systems with low (preferably sub-femtosecond range) timing jitter in a long-term stable arrangement.
To achieve this, the pulse repetition rate of an optical master oscillator implemented as a mode-locked laser is stabilized to a frequency standard or an ultra-low noise microwave oscillator that is clocking the facility. The pulse train is distributed to all critical sub-systems by use of timing stabilized fiber links, i.e. the pulse trains leaving different fiber links are perfectly synchronous. The RF- or optical sub-systems are then synchronized to the pulse trains at the fiber outputs.
Precise transfer of RF signals through fiber links has been demonstrated recently. For timing distribution over the large-scale free electron laser facility, timing stabilized fiber links will be used. If the fiber length is L, one can assume that no length fluctuations are faster than (2 nL)/c, where n is the refractive index of the fiber. Relative fiber expansion by temperature change is typically on the order of 10−7/K, which can be taken out by a length control loop by referencing the back reflected pulse from the fiber end with a later pulse from the mode-locked laser. This concept works to a precision fundamentally limited by the high frequency jitter of the laser from frequency of c/(2 nL) up to the Nyquist frequency, i.e. half of the repetition rate. This jitter should be on the order of a few femtoseconds or below if 10-fs overall jitter needs to be achieved. This puts a serious constraint on the high frequency timing jitter of the optical master oscillator.
It has been shown that the extraction of a microwave signal from an optical pulse train emitted by a mode-locked laser using direct photo-detection is limited in precision by excess phase noise (see E. N. Ivanov, S. A. Diddams, and L. Hollberg, “Analysis of noise mechanisms limiting the frequency stability of microwave signals generated with a femtosecond laser,” IEEE J. Sel. Top. Quant. Elec. 9, 1059-1065 (2003). A. Bartels, S. A. Diddams, C. W. Oates, G. Wilpers, J. C. Bergquist, W. H. Oskay, L. Hollberg, “Femtosecond-laser-based synthesis of ultrastable microwave signals from optical frequency references,” Optics Letters 30, 667-669 (2005)). These publications are incorporated herein by reference in their entirety.
The origin of this excess noise has been identified to be amplitude-to-phase conversion in the photo-detection process, beam-pointing variations, and pulse distortions by photo-detector nonlinearities. In addition to this excess phase noise and timing jitter by photo-detector nonlinearities, the long-term synchronization stability is limited by the temperature dependence of semiconductor photodiodes. Thus, a new synchronization scheme to avoid these problems is highly desirable.